Wilfried Strauch

1995 ERUPTIONS OF CERRO NEGRO VOLCANO, NICARAGUA, AND RISK ASSESSMENT FOR FUTURE ERUPTIONS

Cerro Negro volcano, Nicaragua, continued a 147-yr-long duration of cinder-cone activity with a major eruption in 1995. Small, phreatically driven eruptions began in May 1995 and continued for 79 days. Following a 95 day repose, the main eruption produced 8 × 106 m3 of basalt from Cerro Negro over 13 days of activity and deposited 5 mm of ash in the city of Leon. Although the damage from the 1995 eruptions was fortunately minor, previous tephra falls from Cerro Negro have produced significant crop damage and multiple deaths through building collapse. In spite of its apparent longevity for a historically active cinder cone, Cerro Negro has mass-flow rates typical of arc-related basaltic cinder cone volcanoes. Volcanic hazards beyond 3 km from Cerro Negro consist of tephra falls. Few models are available to calculate tephra-fall risks from basaltic volcanoes such as Cerro Negro, and none have been applied to dispersive cinder cone eruptions. A convective-dispersive model of Suzuki is modified and evaluated using detailed data from the 1995 Cerro Negro eruption and is found to reasonably calculate tephra-fall thickness between 8 and 30 km from the vent. This model is used with detailed data from previous Cerro Negro eruptions in a tephra-fall hazard assessment. Cerro Negro also appears to have had a steady-state eruption rate since about A.D. 1900, which is used to estimate the timing of the next eruption as before A.D. 2006. The potential tephra fall from Cerro Negro in Leon, Nicaragua, is calculated as 2.2 mm/yr until 2006, with 95% confidence that deposits will be <11 cm thick.

Bookshelf faulting in Nicaragua

Oblique subduction at a high rate of convergence along much of the Middle America Trench results in northwest-directed trench-parallel block motion. Accommodation of this motion along northwest-striking dextral strike-slip faults has been postulated; however, in Nicaragua such faults are not well developed. We suggest instead that this motion is accommodated by bookshelf faulting that includes northeast-striking left-lateral faults. We present earthquake epicenter and focal mechanism data and mapped fracture and fault data consistent with this model. Trenchward migration of the volcanic arc since the Miocene and reactivation of northeast-striking Miocene structures may have led to the development of this arc- and trench-normal fault system.

Evidence for static stress changes triggering the 1999 eruption of Cerro Negro Volcano, Nicaragua and regional aftershock sequences

[i] Remarkable evidence of coupling between tectonic and magmatic events emerges from investigation of three tectonic earthquakes, aftershock sequences and eruption of Cerro Negro volcano, Nicaragua in 1999. Here, we explain this coupling through static stress changes following three Mw 5.2 earthquakes. We use focal mechanism solutions to estimate fault system geometry and magnitude of slip from these events, which are then used to calculate the change in minimum horizontal principal stress (σ 3 ) for the region and the change in Coulomb failure stress on optimally oriented fault planes. Results of these simulations indicate that σ 3 was reduced by ∼0.08 MPa and that Coulomb failure stress was raised by 0.001 to 0.2 MPa in the region. A Kolmogorov-Smirnov test demonstrates spatial correlation of Coulomb failure stress changes and triggered seismicity and volcanism, and suggests that these small changes in static stress can trigger subsequent geophysical events under appropriate circumstances. Citation: Dfez, M., P. C. La Femina, C. B. Connor, W. Strauch, and V. Tenorio (2005), Evidence for static stress changes triggering the 1999 eruption of Cerro Negro Volcano, Nicaragua and regional aftershock sequences.

Anomalous Diffuse CO2 Emissions at the Masaya Volcano (Nicaragua) Related to Seismic- Volcanic Unrest

Anomalous changes in the diffuse emission of carbon dioxide within the Masaya caldera have been observed before two seismic events that occurred at 10 and 30xa0km from the observation site. Their epicenters are located, respectively, south of Managua in Las Colinas (4.3 magnitude) and the Xiloa caldera (3.6 magnitude), in 2002 and 2003, recorded by the geochemical station located at El Comalito, Masaya volcano (Nicaragua). Anomalous increases were observed, which occurred around 50 and 8xa0days before the main seismic event that took place in Las Colinas, and 4xa0days before the seismic swarm at the Xiloa caldera, with a maximum CO2 efflux of 9.3 and 10.7xa0kgxa0m−2xa0day−1, respectively. The anomalous CO2 efflux increases remained after filtering with multiple regression analysis was applied to the CO2 efflux time series, which indicated that atmospheric variables, during the first 4xa0months, explained 23xa0% CO2 variability, whereas, during the rest of the time series, CO2 efflux values are poorly controlled with only 6xa0%. The observed anomalies of the diffuse CO2 emission rate might be related to pressure changes within the volcanic–hydrothermal system and/or to geostructural changes in the crust due to stress/strain changes caused before and during the earthquakes’ formation, and seem not to be related to the activity of the main crater of Masaya volcano.